Insulin-like growth factor binding protein 3 gene of golden pompano (TroIGFBP3) promotes antimicrobial immune defense

Insulin-like growth factor binding protein 3 (IGFBP3), an important member of the IGFBP family, plays an important biological role in regulating cellular proliferation, differentiation, growth, apoptosis, and innate immunity. However, studies concerning IGFBP3 in teleosts are very limited and IGFBP3 function remains unclear. In this study, we conducted both in vivo and in vitro functional analyses of an IGFBP3 (TroIGFBP3) from the teleost fish golden pompano (Trachinotus ovatus). TroIGFBP3 is composed of 286 amino acid residues and shares a high amino acid sequence similarity (50.18%-93.71%) with other IGFBP3 sequences in humans and teleosts. TroIGFBP3 was widely distributed in various tissues, with the highest expression in the liver. TroIGFBP3 expression was significantly upregulated following Vibrio harveyi infection. The results of in vitro assays showed that TroIGFBP3 could stimulate macrophage activation and promote peripheral blood leukocytes (PBLs) proliferation. Meanwhile, TroIGFBP3 overexpression significantly inhibited bacterial infection in fish tissues, whereas TroIGFBP3 knockdown resulted in increased bacterial dissemination and colonization in golden pompano tissues in vivo. Furthermore, recombinant TroIGFBP3 could inhibit cellular proliferation and promote apoptosis of mouse tumor cells. Taken together, these results indicated that TroIGFBP3 plays a significant role in innate antibacterial immunity and provides a theoretical foundation for investigating the function of IGFBP3 in fish immune response.

IGFBP-3/IGFBP-3 Receptor System as an Anti-Tumor and Anti-Metastatic Signaling in Cancer

Insulin-like growth factor binding protein-3 (IGFBP-3) is a p53 tumor suppressor-regulated protein and a major carrier for IGFs in circulation. Among six high-affinity IGFBPs, which are IGFBP-1 through 6, IGFBP-3 is the most extensively investigated IGFBP species with respect to its IGF/IGF-I receptor (IGF-IR)-independent biological actions beyond its endocrine/paracrine/autocrine role in modulating IGF action in cancer. Disruption of IGFBP-3 at transcriptional and post-translational levels has been implicated in the pathophysiology of many different types of cancer including breast, prostate, and lung cancer. Over the past two decades, a wealth of evidence has revealed both tumor suppressing and tumor promoting effects of IGF/IGF-IR-independent actions of IGFBP-3 depending upon cell types, post-translational modifications, and assay methods. However, IGFBP-3′s anti-tumor function has been well accepted due to identification of functional IGFBP-3-interacting proteins, putative receptors, or crosstalk with other signaling cascades. This review mainly focuses on transmembrane protein 219 (TMEM219), which represents a novel IGFBP-3 receptor mediating antitumor effect of IGFBP-3. Furthermore, this review delineates the potential underlying mechanisms involved and the subsequent biological significance, emphasizing the clinical significance of the IGFBP-3/TMEM219 axis in assessing both the diagnosis and the prognosis of cancer as well as the therapeutic potential of TMEM219 agonists for cancer treatment.

Insulin-Like Growth Factor Binding Proteins--an Update

The insulin-like growth factor (IGF) system is essential for normal growth and development, and its perturbation is implicated in a number of diseases. IGF activity is finely regulated by a family of six high-affinity IGF binding proteins (IGFBPs). 1GFBPs usually inhibit IGF actions but may enhance them under certain conditions. Additionally, IGFBPs bind non-IGF ligands in the extracellular space, cell membrane, cytoplasm and nucleus, thereby modulating cell proliferation, survival and migration in an IGF-independent manner. IGFBP activity is regulated by transcriptional mechanisms as well as by post-translational modifications and proteolysis. Understanding the balance between the various actions of IGFBPs in vivo may lead to novel insights into disease processes and possible IGFBP-based therapeutics.

Amyloid beta-mediated epigenetic alteration of insulin-like growth factor binding protein 3 controls cell survival in Alzheimer's disease

Swedish double mutation (KM670/671NL) of amyloid precursor protein (APP) is reported to increase toxic amyloid β (Aβ) production via aberrant cleavage at the β-secretase site and thereby cause early-onset Alzheimer's disease (AD). However, the underlying molecular mechanisms leading to AD pathogenesis remains largely unknown. Previously, our transcriptome sequence analyses revealed global expressional modifications of over 600 genes in APP-Swedish mutant-expressing H4 (H4-sw) cells compared to wild type H4 cells. Insulin-like growth factor binding protein 3 (IGFBP3) is one gene that showed significantly decreased mRNA expression in H4-sw cells. In this study, we investigated the functional role of IGFBP3 in AD pathogenesis and elucidated the mechanisms regulating its expression. We observed decreased IGFBP3 expression in the H4-sw cell line as well as the hippocampus of AD model transgenic mice. Treatment with exogenous IGFBP3 protein inhibited Aβ_1–42- induced cell death and caspase-3 activity, whereas siRNA-mediated suppression of IGFBP3 expression induced cell death and caspase-3 cleavage. In primary hippocampal neurons, administration of IGFBP3 protein blocked apoptotic cell death due to Aβ_1–42 toxicity. These data implicate a protective role for IGFBP3 against Aβ_1–42-mediated apoptosis. Next, we investigated the regulatory mechanisms of IGFBP3 expression in AD pathogenesis. We observed abnormal IGFBP3 hypermethylation within the promoter CpG island in H4-sw cells. Treatment with the DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine restored IGFBP3 expression at both the mRNA and protein levels. Chronic exposure to Aβ_1–42 induced IGFBP3 hypermethylation at CpGs, particularly at loci −164 and −173, and subsequently suppressed IGFBP3 expression. Therefore, we demonstrate that expression of anti-apoptotic IGFBP3 is regulated by epigenetic DNA methylation, suggesting a mechanism that contributes to AD pathogenesis.

Quantification of binding of IGF-1 to BI 836845, a candidate therapeutic antibody against IGF-1 and IGF-2, and effects of this antibody on IGF-1:IGFBP-3 complexes in vitro and in male C57BL/6 mice

IGF-1 and IGF-2 are potent mitogens acting through the IGF-1 receptor (IGF-1R). The importance of the IGF system in neoplasia has been demonstrated in several models, and IGF-1 signaling has become a target for drug development. The drug candidate BI 836845 is a fully human IgG1 ligand-neutralizing antibody that cross-reacts with IGF-1 and IGF-2. It has been shown to reduce both IGF-1R phosphorylation and cellular proliferation in preclinical studies. In rodent studies, administration of BI 836845 leads to large increases in total IGF-1 concentration in serum, despite reduced serum IGF-1 activity as measured by a kinase activation assay. Despite the fact that anti-IGF-ligand antibodies have entered clinical trials, their effect on IGF-binding proteins has not been described. In this report, we developed a novel technique to measure ligand-BI 836845 binding, and we apply it to a mouse model in various contexts. We show that although large increases in total serum IGF-1 levels are observed, the vast majority of ligand is present as a complex with BI 836845, and total serum IGF-binding protein-3 levels are decreased. Finally, we show that BI 836845 treatment induces an increase in GH levels, a finding consistent with attempted compensation at the level of the pituitary. Our results reveal complexities in the physiologic sequelae of BI 836845 administration that have implications for determination of optimal dosing regimens and for development of pharmacodynamic endpoints for clinical trials.

Circulating salmon 41-kDa insulin-like growth factor binding protein (IGFBP) is not IGFBP-3 but an IGFBP-2 subtype

In vertebrates, most circulating insulin-like growth factor (IGF) is bound to multiple forms of IGF-binding proteins (IGFBPs) that differ both structurally and functionally. In mammals, the largest reservoir of IGF in the circulation comes from a large (150kDa) ternary complex comprised of IGF bound to IGFBP-3, which is bound to an acid label subunit (ALS), and this variant of IGFBP is regulated by growth hormone (GH) and feed intake. Although multiple variants of IGFBPs ranging from 20 to 50kDa have been found in fishes, no ternary complex is present and it has been assumed that the majority of circulating IGF is bound to fish IGFBP-3. Consistent with this assumption is previous work in salmon showing the presence of a 41-kDa IGFBP that is stimulated by GH, decreases with fasting and increases with feeding. However, the hypothesis that the salmon 41-kDa IGFBP is structurally homologous to mammalian IGFBP-3 has not been directly tested. To address this issue, we cloned cDNAs for several Chinook salmon IGFBPs, and found that the cDNA sequence of the 41-kDa IGFBP is most similar to that of mammalian IGFBP-2 and dissimilar to IGFBP-3. We found an additional IGFBP (termed IGFBP-2a) with high homology to mammalian IGFBP-2. These results demonstrate that salmon 41-kDa IGFBP is not IGFBP-3, but a paralog of IGFBP-2 (termed IGFBP-2b). Salmon IGFBP-2s are also unique in terms of having potential N-glycosylation sites and splice variants. Additional research on non-mammalian IGFBPs is needed to fully understand the molecular/functional evolution of the IGFBP family and the significance of the ternary complex in vertebrates.

Alterations of insulin-like growth factor binding protein 3 (IGFBP-3) glycosylation in patients with breast tumours

OBJECTIVES

Insulin-like growth factor binding protein-3 (IGFBP-3) is an important modulator of development and progression of breast cancer as it regulates the amount of free, physiologically active IGF-I and IGF-II. Changes in the glycosylation pattern within IGFBP-3 may affect its interaction with ligands. The aim of this study was to investigate whether such changes occur during disease progression.

DESIGN AND METHODS

IGFBP-3 in serum samples from healthy women and from women with breast tumours was characterised in terms of its concentration (IRMA), glycosylation moiety (lectin-affinity chromatography) and distribution of molecular species (immunoblotting).

RESULTS

In patients with benign tumours the concentration and carbohydrate content of IGFBP-3 was unaltered compared to healthy women. In patients with malignant tumours in most cases these two parameters were unchanged, but there were women whose concentration of IGFBP-3 was reduced and its structure was altered. In non-surviving cancer patients the concentration of IGFBP-3 was significantly reduced and these molecules contained a greater amount of biantennary complex type N-glycans having more mannose, fucose, bisecting GlcNAc and terminal sialic acid residues.

CONCLUSION

Our results showed that breast cancer progression causes alterations of IGFBP-3 glycosylation. The extent of changes increases with breast cancer severity.

Activity of human pregnancy insulin-like growth factor binding protein-3: determination by reconstituting recombinant complexes

During pregnancy, IGF binding protein-3 (IGFBP-3) is completely proteolyzed to fragments with low affinities for IGFs but appears to transport IGFs normally in high-molecular-mass complexes. We previously reported that synthetic isolated amino- and carboxyl-terminal domains of IGFBP-3 cooperate to bind IGFs, and we investigated whether this is the mechanism whereby proteolyzed IGFBP-3 fragments bind IGFs normally in pregnancy serum. Two fragments of IGFBP-3 have been isolated from pregnancy serum, one with the same N-terminal sequence as intact IGFBP-3 (GASSG) and the other with an N-terminal sequence (160)KVDYE. Recombinant forms of these proteins, IGFBP-3(1-159) and IGFBP-3(160-264), have been synthesized and characterized, demonstrating that although the fragments individually have greatly reduced affinity for IGF complex formation, when combined they cooperate to form complexes with IGF with or without the acid-labile subunit, inhibit IGF transport across endothelial cell monolayers and inhibit IGF-I-induced IGF type I receptor phosphorylation. It is proposed that proteolysis of IGFBP-3 into two discrete complementary fragments does not significantly increase IGF bioavailability, consistent with previous findings that proteolyzed IGFBP-3 in pregnancy serum is functionally normal and circulates as part of the IGF ternary complexes.

Effects of fasting on IGF-I, IGF-II, and IGF-binding protein mRNA concentrations in channel catfish (Ictalurus punctatus)

The effects of fasting on insulin-like growth factor (IGF)-I, IGF-II, and IGF-binding protein (IGFBPs) mRNA in channel catfish were examined. Fed control fish (Fed) were compared to fish that had been fasted for 30 d followed by 15 d of additional feeding (Restricted). Sequence alignment and similarity to orthologous proteins in other vertebrates provided structural evidence that the 3 catfish sequences identified in the present research were IGFBP-1, -2, and -3. Prolonged fasting (30 d) reduced body weight approximately 60% (P<0.001) and decreased IGF-I mRNA in the liver and muscle (P<0.01). Fifteen days of re-feeding restored concentrations of hepatic and muscle IGF-I mRNA. Liver IGF-II mRNA was not affected by fasting but was increased 2.2-fold after 15 d of re-feeding (P<0.05). Abundance of muscle IGF-II mRNA was similar between the fed control group and the restricted group throughout the experimental period. Fasting also increased liver IGFBP-1 mRNA (P<0.05) and decreased IGFBP-3 mRNA (P<0.01), whereas abundance of IGFBP-2 mRNA was not significantly affected. Interestingly, re-feeding for 15 d did not restore concentrations of IGFBP-1 and IGFBP-3 mRNA relative to fed control concentrations. The IGF results suggest that IGF-I and IGF-II are differently regulated by nutritional status and probably have a differential effect in promoting muscle growth during recovery from fasting. Similar to mammals, IGFBP-1 mRNA in catfish is increased during catabolism, whereas IGFBP-3 mRNA is decreased during inhibited somatic growth. The IGFBP results provide additional evidence of the conserved nature of the IGF-IGFBP-growth axis in catfish.

Induction of apoptosis in human prostate cancer cells by insulin-like growth factor binding protein-3 does not require binding to retinoid X receptor-alpha

IGF binding protein (IGFBP)-3 can induce apoptosis in human prostate cancer cells directly without sequestering IGF-I and -II. The molecular mechanisms responsible for the IGF-independent actions of IGFBP-3 remain unclear. IGFBP-3, a secreted protein, can be internalized and translocate to the nucleus. It binds to the nuclear retinoid X receptor (RXR)-alpha. Binding to RXR-alpha has been proposed to be required for IGFBP-3 to induce apoptosis. The present study tests this hypothesis in the PC-3 human prostate cancer cell line. PC-3 cells express RXR-alpha, and apoptosis is induced by incubation with RXR-specific ligand. A COOH-terminal region in IGFBP-3 (residues 215-232) contains a nuclear localization signal, and binding domains for RXR-alpha and heparin (HBD). Different combinations of the 11 amino acids in this region that differ from IGFBP-1, a related IGFBP, which does not localize to the nucleus or bind RXR-alpha, were mutated to the IGFBP-1 sequence. By confocal imaging, mutation of residues 228-KGRKR-232 in nonsecreted IGFBP-3 diminished its nuclear localization. IGFBP-3 binding to glutathione S-transferase-RXR-alpha only was lost when all 11 sites were mutated (HBD-11m-IGFBP-3). Expressed nuclear RXR-alpha did not transport cytoplasmic IGFBP-3 nuclear localization signal mutants that can bind RXR-alpha to the nucleus even after treatment with RXR ligand. Expressed HBD-11m-IGFBP-3 still induced apoptosis in PC-3 cells in an IGF-independent manner as determined by flow cytometric analysis of Annexin V staining. We conclude that in PC-3 cells, RXR-alpha is not required for the nuclear translocation of IGFBP-3 and that IGFBP-3 can induce apoptosis in human prostate cancer cells without binding RXR-alpha.

Molecular basis of the interaction between IGFBP-3 and retinoid X receptor: role in modulation of RAR-signaling

IGFBP-3 interacts with the retinoid X receptor-alpha (RXRalpha) and retinoic acid receptor-alpha (RARalpha) and thereby interferes with the formation of RXR:RAR heterodimers. Here we identify the domains in RXRalpha and IGFBP-3 that participate in this interaction. When different regions of RXRalpha were expressed independently, we found that only the DNA-binding domain (C-domain) bound IGFBP-3. Residues in the second Zn-finger loop (Gln49, Arg52), which contribute to C-domain dimerization on DR1 response elements, proved essential to IGFBP-3 binding. In complementary studies, we found that residues within the N-terminal domain of IGFBP-3 (Thr58, Arg60) and motifs in its C-terminal domain ((220)LysLysLys, (228)LysGlyArgLysArg) were required for interaction with RXRalpha and RARalpha. Unlike wild-type IGFBP-3, the non-retinoid receptor-binding mutants of IGFBP-3 were unable to attenuate all-trans-retinoic acid-induced transactivation of the RAR response element by RXR:RAR heterodimers. We conclude that residues in both the N- and C-terminal domains of IGFBP-3 are involved in binding the retinoid receptors, and that this interaction is essential to the modulation of RAR-signaling by IGFBP-3.

Sample pre-treatment determines the clinical usefulness of acid-labile subunit immunoassays in the diagnosis of growth hormone deficiency and acromegaly

OBJECTIVE

The usefulness of measuring the GH-dependent acid-labile subunit (ALS) in the management of GH deficiency (GHD) and acromegaly remains in question and is investigated in this study, comparing several different immunoassays for ALS.

METHOD

We compared the diagnostic accuracy of a commercially available polyclonal Ab-based ELISA with SDS pre-treatment (SDS-ELISA) with a monoclonal Ab-based immunofluorometric assay, using two unfolding methods (urea (UREA) and Glycine-HCl (Gly)). The corresponding molecular weight (MW) of ALS and IGFBP-3 immunoreactivity was determined. The clinical usefulness of each assay was examined in adult GH disorders.

RESULTS

ALS was lower in GHD and higher in acromegaly using all assays. In GHD, UREA had higher sensitivity and specificity than SDS-ELISA (59 and 69% versus 41 and 51% respectively). In acromegaly, sensitivity and specificity was 94 and 87% for UREA, 81 and 36% for Gly, and 44 and 44% for SDS-ELISA. After UREA, immunoreactivity for ALS and IGFBP-3 eluted at their predicted free MW using size-exclusion chromatography, whereas ALS immunoreactivity in SDS (300-600 kDa) and Gly (250-500 kDa) was at a high apparent MW consistent with aggregation.

CONCLUSION

The diagnostic accuracy of ALS varies with assay choice and pre-treatment modality. UREA, which results in migration of ALS at the expected MW on a sizing column, has the highest specificity and sensitivity. Thus, if measured in an assay in which ALS is unfolded without aggregation, ALS is a clinically highly useful parameter for the assessment of GH.

The aminoterminal insulin-like growth factor (IGF) binding domain of IGF binding protein-3 cannot be functionally substituted by the structurally homologous domain of CCN3

IGF binding proteins (IGFBPs) are a family of structurally homologous proteins that bind IGFs with high affinities and can modulate IGF activity. The IGF binding site has been shown to comprise residues in both the aminoterminal and carboxyterminal domains. In recent years several proteins including members of the CCN (connective tissue growth factor, Cyr61, and nephroblastoma overexpressed) family were recognized as having structural homology in their aminoterminal domains to the IGFBPs. Despite their low or undetectable IGF binding ability, a proposal was made to rename them as IGFBP-related proteins. To test whether the aminoterminal domain of a CCN protein can fulfill the high-affinity IGF binding function of an IGFBP, we created a chimera in which the aminoterminal domain of IGFBP-3 was substituted with the aminoterminal domain of CCN3 (previously known as Nov). The CCN3-IGFBP-3 chimera bound IGFs and inhibited IGF activity very weakly, similar to CCN3 itself. Although structurally similar, the aminoterminal domain of CCN3 is unable to replace the aminoterminal domain of IGFBP-3 in forming a high-affinity IGF-binding site. These results argue against a direct role of CCN3 in the regulation of IGF bioavailability and indicate that the nomenclature of IGFBP-related proteins (which implies functional relationship to the classical IGFBPs) is inappropriate for CCN proteins.

Structural characterization and binding properties of the hemopexin-like domain of the matrixmetalloproteinase-19

The matrixmetalloproteinase-19 (MMP-19) belongs to the superfamily of the zinc-dependent endopeptidases, which are secreted by cells and are involved in the remodeling of the extracellular matrix. The full-length protein consists of a signal peptide, a propeptide, a catalytic domain and a C-terminal hemopexin-like domain. For other members of this superfamily, the hemopexin-like domain has been described to be involved in substrate recognition. In this study, the hemoxpexin domain of MMP-19 was expressed in Escherichia coli, refolded, and purified. For structural characterization, circular dichroism and NMR spectroscopy were used. We show that the hemopexin-like domain of MMP-19 is able to bind calcium and this binding induces a conformational change and an increase in the thermal stability of the domain. MMP-19 promotes proliferation of keratinocytes by cleaving the insulin-like-growth factor binding protein-3, thereby causing the release of IGF-1, which is a potent growth factor for these cells. By plasmon resonance experiments, we show that the isolated hemopexin-like domain is able to bind to the insulin-like-growth factor binding protein-3. These results provide a basis for further structural investigations that could be used for the rational design of potential agonists and antagonists.

Several acidic amino acids in the N-domain of insulin-like growth factor-binding protein-5 are important for its transactivation activity

Insulin-like growth factor-binding protein (IGFBP)-5 is a secreted protein that binds to IGFs and modulates IGF actions. IGFBP-5 is also found in the nuclei of cultured cells and has transactivation activity. Here we report the nuclear localization of endogenous IGFBP-5 in mouse embryonic skeletal cells. Chromatin immunoprecipitation experiments indicated that IGFBP-5 interacts with the nuclear histone-DNA complex. Using a series of deletion mutants, the transactivation domain of IGFBP-5 was mapped to its N-terminal region. Intriguingly, the transactivation activity of IGFBP-5 is masked by negative regulatory elements located in the L- and C-domains. Among the other IGFBPs, the N-domains of IGFBP-2 and -3 also had strong transactivation activity, whereas those of IGFBP-1 and -6 had no activity. The IGFBP-4 N-domain had modest activity. Sequence analysis revealed several amino acids in the IGFBP-5 N-domain that are not present in IGFBP-1. The activities of mutants in which these residues were changed to the corresponding IGFBP-1 sequence were determined. Mutations that changed acidic residues to neutral residues (e.g. E8A, D11S, E12A, E30S/P31A, E43L, and E52A) or a polar to a basic residue (e.g. Q56R) significantly reduced transactivation activity. The E8A/D11S/E12A triple mutant and E52A/Q56R double mutants showed further reduced activity. The combinatory mutants had essentially no transactivation activity. Taken together, our results indicate that there are several conserved residues in the IGFBP-5 N-terminal region that are critical for transactivation and that IGFBP-2 and -3 also have strong transactivation activity in their N-domains.

Enhanced insulin-like growth factor-I (IGF-I) cell association at reduced pH is dependent on IGF binding protein-3 (IGFBP-3) interaction

The cellular microenvironment impacts how signals are transduced by cells and plays a key role in tissue homeostasis. Although pH is generally well regulated, there are a number of situations where acidosis occurs and our work addresses how low pH impacts cell association of insulin-like growth factor-I (IGF-I) in the presence of IGF binding protein-3 (IGFBP-3). We have previously shown that IGF-I cell binding was enhanced in the presence of IGFBP-3 at low pH and now show that this binding is IGFBP-mediated as it is inhibited by Y60L-IGF-I, a mutant with reduced affinity for the IGF receptor (IGF-IR), and unaffected by insulin, which binds but not IGFBPs. Using surface plasmon resonance (SPR), we show that direct binding between IGF-I and IGFBP-3 is pH sensitive. Despite this, the key step in the process appears to be IGFBP-3 cell surface association as Long-R(3)-IGF-I, a mutant with reduced affinity for IGFBPs, shows a similar increase in cell association at pH 5.8 in the presence of IGFBP-3 but does not exhibit pH-dependent binding by SPR. Further, analysis indicates a large increase in low-affinity binding sites for IGF-I in the presence of IGFBP-3 and an elimination of IGF-I enhanced binding when a non-cell associating mutant of IGFBP-3 is added in place of IGFBP-3. That the IGFBP-3-mediated binding localizes IGF-I away from IGF-IR is suggested by triton-solubility testing and indicates additional complexities to IGF-I regulation by IGFBP-3. Identifying the pH-dependent binding partner(s) for IGFBP-3 is a necessary next step in deciphering this process.

Up-regulation of insulin-like growth factor binding protein-3 by apigenin leads to growth inhibition and apoptosis of 22Rv1 xenograft in athymic nude mice

Epidemiological studies suggest that increased intake of fruits and vegetables may be associated with a reduced risk of prostate cancer. Apigenin (4', 5, 7,-trihydroxyflavone), a common dietary flavonoid abundantly present in fruits and vegetables, has shown remarkable anti-proliferative effects against various malignant cell lines. However, the mechanisms underlying these effects remain to be elucidated. We investigated the in vivo growth inhibitory effects of apigenin on androgen-sensitive human prostate carcinoma 22Rv1 tumor xenograft subcutaneously implanted in athymic male nude mice. Apigenin was administered to mice by gavage at doses of 20 and 50 mug/mouse/day in 0.2 ml of a vehicle containing 0.5% methyl cellulose and 0.025% Tween 20 in two different protocols. In the first protocol, apigenin was administered for 2 wk before inoculation of tumor and was continued for 8 wk, resulting in significant inhibition of tumor volume by 44 and 59% (P<0.002 and 0.0001), and wet weight of tumor by 41 and 53% (P<0.05), respectively. In the second protocol, administration of apigenin began 2 wk after tumor inoculation and continued for 8 wk; tumor volume and wet weights of tumor were reduced by 39 and 53% (P<0.01 and 0.002) and 31 and 42% (P<0.05), respectively. The tumor inhibitory effect of apigenin was more pronounced in the first protocol of extended treatment, which was associated with increased accumulation of human IGFBP-3 in mouse serum along with significant increase in IGFBP-3 mRNA and protein expression in tumor xenograft. Apigenin intake by these mice also resulted in simultaneous decrease in serum IGF-I levels and induction of apoptosis in tumor xenograft. Importantly, tumor growth inhibition, induction of apoptosis, and accumulation of IGFBP-3 correlated with increasing serum and tumor apigenin levels. In both studies, animals did not exhibit any signs of toxicity or reduced food consumption. In cell culture studies, apigenin treatment resulted in cell growth inhibition and induction of apoptosis, which correlated with increased accumulation of IGFBP-3 in culture medium and cell lysate. These effects were associated with significant reduction in IGF-I secretion; inhibition of IGF-I-induced cell cycle progression and insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, along with an increase in sub-G1 peak by apigenin. Further, treatment of cells with IGFBP-3 antisense oligonucleotide reversed these effects and attenuated apigenin-mediated inhibition of IRS-1 phosphorylation conferring inhibitory effects of apigenin on IGF-signaling. This study presents the first evidence that the in vitro and in vivo growth inhibitory effects of apigenin involve modulation of IGF-axis signaling in prostate cancer.

Ligand rebinding: self-consistent mean-field theory and numerical simulations applied to surface plasmon resonance studies

Rebinding of dissociated ligands from cell surface proteins can confound quantitative measurements of dissociation rates important for characterizing the affinity of binding interactions. This can be true also for in vitro techniques such as surface plasmon resonance (SPR). We present experimental results using SPR for the interaction of insulin-like growth factor-I (IGF-I) with one of its binding proteins, IGF binding protein-3 (IGFBP-3), and show that the dissociation, even with the addition of soluble heparin in the dissociation phase, does not exhibit the expected exponential decay characteristic of a 1:1 binding reaction. We thus consider the effect of (multiple) rebinding events and, within a self-consistent mean-field approximation, we derive the complete mathematical form for the fraction of bound ligands as a function of time. We show that, except for very low association rate and surface coverage, this function is nonexponential at all times, indicating that multiple rebinding events strongly influence dissociation even at early times. We compare the mean-field results with numerical simulations and find good agreement, although deviations are measurable in certain cases. Our analysis of the IGF-I-IGFBP-3 data indicates that rebinding is prominent for this system and that the theoretical predictions fit the experimental data well. Our results provide a means for analyzing SPR biosensor data where rebinding is problematic and a methodology to do so is presented.

Mecasermin Rinfabate: Insulin-Like Growth Factor-I/Insulin-Like Growth Factor Binding Protein-3, Mecaserimin Rinfibate, rhIGF-I/rhIGFBP-3

Insmed is developing mecasermin rinfabate, a recombinant complex of insulin-like growth factor-I (rhIGF-I) and binding protein-3 (rhIGFBP-3) [insulin-like growth factor-I/insulin-like growth factor binding protein-3, rhIGF-I/rhIGFBP-3, SomatoKine], for a number of metabolic and endocrine indications. In the human body, IGF-I circulates in the blood bound to a binding protein-3 (IGFBP-3), which regulates the delivery of IGF-I to target tissues, and particular proteases clip them apart in response to stresses and release IGF-I as needed. IGF-I, a naturally occurring hormone, is necessary for normal growth and metabolism. For the treatment of IGF-I deficiency, it is desirable to administer IGF-I bound to IGFBP-3 to maintain the normal equilibrium of these proteins in the blood. Mecasermin rinfabate (rhIGF-I/rhIGFBP-3) mimics the effects of the natural protein complex in the bloodstream and would augment the natural supply of these linked compounds. The most advanced indication in development of mecasermin rinfabate is the treatment of severe growth disorders due to growth hormone insensitivity syndrome (GHIS), also called Laron syndrome. GHIS is a genetic condition in which patients do not produce adequate quantities of IGF because of a failure to respond to the growth hormone signal. This results in a slower growth rate and short stature. Mecasermin rinfabate also has potential as replacement therapy for IGF-I, which may become depleted in indications such as major surgery, organ damage/failure, traumatic injury, cachexia and severe burn trauma. It also has potential for the treatment of osteoporosis. Mecasermin rinfabate was developed by Celtrix using its proprietary recombinant protein production technology. Subsequently, Celtrix was acquired by Insmed Pharmaceuticals on 1 June 2000. Insmed and Avecia of the UK have signed an agreement for manufacturing mecasermin rinfabate and its components, rhIGF-1 and rhIGFBP-3. CGMP clinical production of mecasermin rinfabate and its components will be carried out in Avecia's Advanced Biologics Centre, Billingham, UK, which manufactures recombinant-based medicines and vaccines at the capacity of up to 1000L. In April 2004, Insmed announced that it acquired a lease to operate the manufacturing facility formerly operated by Baxter for the commercial production of SomatoKine in Boulder, CO, USA. With the two manufacturing facilities for SomatoKine, Insmed plans to meet the development and commercial demands for the product over the next several years. In its 2003 Form-10K, Insmed announced plans to conduct comparative studies with the previously used drug substance and the new substance produced by Avecia. The comparative data will be included in the regulatory filing for mecasermin rinfabate. Mecasermin rinfabate was originally licensed to Welfide for Japan. On 1 October 2001, Welfide Corporation merged with Mitsubishi-Tokyo Pharmaceuticals to form Mitsubishi Pharma Corporation. The new company is a subsidiary of Mitsubishi Chemical. In October 2004, Insmed announced that Tzamal Pharma has been granted exclusive distribution and marketing rights for mecasermin rinfabate in certain Middle Eastern territories including Israel. Tzamal Pharma also acquired exclusive rights to Insmed's named patient programme for the agent in these territories. Tzamal Pharma intends to begin the appropriate registration activities for mecasermin rinfabate in the treatment of children with growth hormone-insensitivity syndrome. This pivotal, 12-month, multicentre, open-label trial in 30 children with GHIS was initiated in June 2003 and was designed to evaluate the safety and efficacy of the agent in prepubescent children with GHIS. The 6-month endpoint data analysis showed that mecasermin rinfabate given as a once-daily injection was safe and well tolerated. The agent demonstrated a significant increase in height velocity in children with GHIS similar to that observed by Pfizer in their pivotal study with twice-daily injections of rhIGF-I. The full results from the pivotal trial are expected in 2005. In April 2003 Insmed initiated a named patient programme in Europe that will make available mecasermin rinfabate for the treatment of GHIS-Laron syndrome. The treatment of patients was initiated in Scandinavia, with authorisation pending in several other European countries. Mecasermin rinfabate will be made available to those GHIS patients who, in the opinion of their doctor, may benefit from IGF-I therapy. At precommercial scale quantities, the drug will be available on a limited basis.A phase II dose-ranging study in children with GHIS was completed at Saint Bartholomew's and the Royal London School of Medicine, London, UK. A single dose of mecasermin rinfabate delivered the same amount of IGF-1 as two daily injections of unbound IGF-1. No adverse events were reported. Insmed has acquired an exclusive licence to Pharmacia's regulatory filings concerning yeast-derived insulin-like growth factor 1 (IGF-1). These filings were used by Pharmacia to receive marketing approvals in several European countries and also in the IND application with the US FDA. Insmed believes that this licence will facilitate the development of mecasermin rinfabate for the treatment of children with GHIS. In January 2003, Insmed announced positive results from a double-blind, placebo-controlled, dose-ranging study of mecasermin rinfabate in adolescent patients with type 1 diabetes receiving insulin therapy. The study was conducted at the University of Cambridge, Cambridge, UK, under supervision of Prof. D. Dunger. The researchers from The Robarts Research Institute and the University of Western Ontario, Canada (leading investigator T.L. Delovitch, the Sheldon H. Weinstein scientist in Diabetes at the University of Western Ontario) have found that mecasermin rinfabate complex was significantly more effective than IGF-1 in reducing the severity of insulitis, beta cell destruction and delaying the onset of type 1 diabetes. The study was supported by grants from Canadian Institutes of Health and the Juvenile Diabetes Research Foundation. Insmed plans to initiate large-scale phase II clinical studies in this indication. At the BIO 2004 Annual International Convention (BIO-2004) in June 2004, Insmed announced that it has received a grant from the US National Institutes of Health (NIH)/Muscular Dystrophy Association (MDA) worth USD $6.5 million to investigate the efficacy of mecasermin rinfabate for the treatment of myotonic dystrophy. It has also been granted orphan drug status for the treatment of GHIS-Laron syndrome in the US and Europe. In December 2003, Insmed announced that mecasermin rinfabate was designated orphan drug status by the FDA for the treatment of extreme insulin resistance. This provides Insmed with 7 years of market exclusivity following approval of mecasermin rinfabate for this indication. Insmed has received orphan drug designation for mecasermin rinfabate in the treatment of extreme insulin resistance in Europe (October 2004). In November 2004, Insmed was granted the European patent EP1183042 entitled "Methods for Treating Diabetes". This patent corresponds with the US patent US 6,040,292 also entitled "Methods for Treating Diabetes". Both patents cover type 1 and type 2 diabetes mellitus and insulin resistant diabetes including type A insulin resistance (the least severe form of extreme insulin resistance syndromes). In January 2004, Insmed obtained a non-exclusive licence to the patents for use of IGF-I for the treatment of extreme or severe insulin-resistant diabetes from Fujisawa Pharmaceutical. Insmed will have worldwide rights in territories (excluding Japan) with existing valid patent claims including the US and Europe. Insmed holds 28 US issued or allowed patents for the composition, production, antibodies and methods of use of mecasermin rinfabate. These US patents expire at various times between the years 2010 and 2019. Insmed through their lawyers filed its defense and counterclaim to the alleged patent infringement brought by Tercica against Insmed in the London High Court of Justice. Insmed asserted that it did not infringe any valid patent claims as none of the claims of the patent were patentable because the subject matter was not new. Insmed also stated that the patent did not involve an inventive step, did not have capability of industrial application and had no clear description of the invention so that invention can be performed by the person skilled in the art. Insmed is seeking revocation of the patent on these grounds.

Role of N- and C-terminal residues of insulin-like growth factor (IGF)-binding protein-3 in regulating IGF complex formation and receptor activation

Insulin-like growth factor-binding protein-3 (IGFBP-3), the major IGFBP in the circulation, sequesters IGF in a stable ternary complex with the acid-labile subunit. The high affinity IGF-binding site is proposed to reside within an N-terminal hydrophobic domain in IGFBP-3, but C-terminal residues have also been implicated in the homologous protein IGFBP-5. We have mutated in various combinations Leu(77), Leu(80), and Leu(81) in the N terminus and Gly(217) and Gln(223) in the C terminus of IGF-BP-3. All mutants retained immunoreactivity toward a polyclonal IGFBP-3 antibody, whereas IGF ligand blotting showed that all of the mutants had reduced binding to IGFs. Both solution IGF binding assays and BIAcore analysis indicated that mutations to the N-terminal region caused greater reduction in IGF binding activity than C-terminal mutations. The combined N- and C-terminal mutants showed undetectable binding to IGF-I but retained <10% IGF-II binding activity. Reduced ternary complex formation was seen only in mutants that had considerably reduced IGF-I binding, consistent with previous studies indicating that the binary IGF.IGFBP-3 complex is required for acid-labile subunit binding. Decreased IGF binding was also reflected in the inability of the mutants to inhibit IGF-I signaling in IGF receptor overexpressing cells. However, when present in excess, IGFBP-3 analogs defined as non-IGF-binding by biochemical assays could still inhibit IGF signaling. This suggests that residual binding activity of IGFBP-3 mutants may still be sufficient to inhibit IGF biological activity and questions the use of such analogs to study IGF-independent effects of IGFBP-3.

Matrix metalloproteinase-7 facilitates insulin-like growth factor bioavailability through its proteinase activity on insulin-like growth factor binding protein 3

Matrix metalloproteinase-7 (MMP-7) secreted by cancer cells has been implicated classically in the basement membrane destruction associated with tumor cell invasion and metastasis. Recent epidemiologic studies have established a correlation between high levels of circulating insulin-like growth factor (IGF) and low levels of IGF binding protein 3 (IGFBP-3), and relative risk of developing colon, breast, prostate, and lung cancer, which are known to produce MMP-7. In this study, IGFBP-3 was assessed as a candidate for the physiologic substrate of MMP-7. MMP-7 proteolysis generated four major fragments (26 kDa, 17 kDa, 15.5 kDa, and 15.5 kDa), and two cleavage sites were identified: one at the site of hydrolysis of the K(144)-I(145) peptide bond and one at the R(95)-L(96) peptide bond. The former site is different from the previously reported site of cleavage of IGFBP-3 by other proteases. Addition of IGFBP-3 inhibited IGF-I-mediated IGF type 1 receptor (IGF-IR) phosphorylation and activation of the downstream molecule Akt in BALB/c 3T3 fibroblasts overexpressing human IGF-IR (3T3-IGF-IR) and in two human colon cancer cell lines (COLO201 and HT29). Coincubation of the IGF-I/IGFBP-3 complex with MMP-7 restored IGF-I-mediated IGF-IR phosphorylation and activation of Akt in these cell lines. The IGF-I signal recovered by MMP-7 protected against apoptosis induced by anoikis in 3T3-IGF-IR cells. These results indicate that MMP-7 proteolysis of IGFBP-3 plays a crucial role in regulating IGF-I bioavailability, thereby promoting cell survival. This mechanism may contribute to the tumorigenesis of MMP-7-producing IGF-IR-expressing tumors in the primary site and to organ-specific metastasis in a paracrine manner.

Cellular internalization of insulin-like growth factor binding protein-3: distinct endocytic pathways facilitate re-uptake and nuclear localization

Insulin-like growth factor binding protein-3 (IGFBP-3) is well established as a growth-inhibitory, apoptosis-inducing secreted molecule that acts via insulin-like growth factor (IGF)-independent as well as IGF-dependent pathways. Nuclear localization of IGFBP-3 has been observed and nuclear binding partners for IGFBP-3 demonstrated. However, little is known about the mechanism of IGFBP-3 internalization. We hypothesized that IGFBP-3 is first secreted then taken up again into cells and that its internalization could occur via binding to transferrin or caveolin. Incubation of cells with an IGFBP-3-neutralizing antibody demonstrated that nuclear translocation of endogenous IGFBP-3 requires IGFBP-3 secretion and re-uptake. Nuclear localization of exogenously added IGFBP-3 was rapid, occurring within 15 min, inhibited by co-incubation and extracellular sequestration with IGF-I, and dependent on the transferrin-binding C-terminal peptide region of IGFBP-3. Co-immunoprecipitation assays confirmed that IGFBP-3 binds transferrin but not directly to the transferrin receptor (TfR1); however, transferrin binds TfR1 and a ternary complex is formed. Specific binding to caveolin scaffolding docking sequence was confirmed utilizing radiolabeled IGFBP-3. Blocking TfR1-mediated endocytosis prevents both endogenous and exogenous IGFBP-3 re-uptake and inhibitors of caveolae formation also retard IGFBP-3 nuclear entry. Co-treatment with anti-transferrin receptor antibody and cholesterol depletion agents completely abolished endogenous and exogenous IGFBP-3 uptake. Suppression of IGFBP-3 internalization by TfR1 blockade inhibited IGFBP-3-induced apoptosis. Together, these data indicate that the actions of IGFBP-3 are mediated by internalization via distinct endocytic pathways.

Insulin-like growth factor-independent effects mediated by a C-terminal metal-binding domain of insulin-like growth factor binding protein-3

Insulin-like growth factors (IGFs) play a central role in the integration of proliferative and survival responses of most mammalian cell types. IGF-binding protein-3 (IGFBP-3) influences IGF action directly as a carrier of IGFs but also modulates these actions indirectly via independent mechanisms involving interactions with plasma, extracellular matrix and cell surface molecules, conditional proteolysis, cellular uptake, and nuclear transport. Here we demonstrate that a short C-terminal metal-binding domain (MBD) of IGFBP-3 mediates binding to metals. MBD epitopes, sequestered in the intact molecule, are unmasked by incubation in the presence of ferrous (but not ferric or zinc) ions. An isolated 14-mer MBD peptide triggered apoptotic effects in stressed HEK293 cells as effectively as IGFBP-3. The MBD, which encompasses a nuclear localization sequence and an adjacent putative caveolin-binding sequence, mobilizes rapid cellular uptake and nuclear localization of unrelated proteins such as green fluorescent protein and streptavidin-horseradish peroxidase conjugate. Metal ions stimulate MBD-mediated cellular/nuclear uptake in vivo. Cross-linking studies showed a direct physical interaction of MBD with integrins alphav and beta1, caveolin-1, and transferrin receptor. MBD-mediated protein mobilization and pro-apoptotic effects are inhibited by nystatin but not chlorpromazine, suggesting an involvement of caveolar-mediated endocytosis. However, MBD effects are inhibited by antibodies to transferrin receptor or integrins. These results are discussed with particular reference to the cell target specificity of IGFBP-3 in disease processes such as cancer and atherosclerosis.

Amino- and carboxyl-terminal fragments of insulin-like growth factor (IGF) binding protein-3 cooperate to bind IGFs with high affinity and inhibit IGF receptor interactions

Both the amino-terminal and carboxyl-terminal domains of IGF binding protein (IGFBP)-3 are believed to contribute to high-affinity IGF binding. To investigate cooperativity in IGF binding by these domains, we expressed IGFBP-3 fragments 1-88 (NBP-3) and 185-264 (CBP-3) as FLAG and hexahistidine-tagged fusion proteins, respectively. IGF-I and IGF-II bound to NBP-3 poorly and to CBP-3 with moderate affinities, approximately 1 liter/nmol. Coincubating the fragments in equimolar concentrations caused a significant cooperative increase in IGF binding, demonstrated by immunoprecipitation with IGFBP-3, FLAG, or hexahistidine antibodies. Equimolar NBP-3 + CBP-3 bound IGF-II with an affinity (12.2 liter/nmol) only 4-fold lower than that of the IGFBP-3-IGF-II complex and IGF-I with an affinity (3.2 liter/nmol) 13-fold lower than IGFBP-3-IGF-I. Heterotrimeric complexes of NBP-3, CBP-3, and IGF, also demonstrated by affinity labeling, bound acid-labile subunit poorly. Coprecipitation assays with iodinated NBP-3 or CBP-3 indicated that the fragments cannot interact unless IGF is also present. Complexing with NBP-3 + CBP-3 inhibited IGF stimulation of type 1 IGF receptor activity and IGF-II binding to the type II receptor. This study demonstrates that isolated amino-terminal and carboxyl-terminal domains of IGFBP-3 cooperate in the presence of IGFs to form high-affinity complexes that retain the ability to block IGF activity.

Evidence implicating a mid-region sequence of IGFBP-3 in its specific IGF-independent actions

Insulin-like growth factor binding protein-3 (IGFBP-3) is one of six high affinity-binding proteins that share a common function in regulating the bioavailability of the insulin-like growth factors. The six binding proteins have highly conserved C- and N-terminals that are essential to this function. Additionally, they all have specific functions on cellular homeostasis independent to the regulation of the insulin-like growth factors. It has previously been shown that insulin-like growth factor binding protein-3 can accentuate UV-induced apoptosis in a human carcinoma cell line. Using the KYSE 190 oesophageal carcinoma cell line we have demonstrated that a 15 amino acid (aa) peptide that lies within the mid-region of the protein can mimic the effect of the intact protein. This region contains the serine residues Ser(111) and Ser(113). Using two protocols, we modified these serine residues and have shown that both phosphorylation and derivatization of IGFBP-3 can negate the accentuation of UV-induced cell death. These three independent pieces of evidence support the hypothesis that the variable mid-region is responsible for the specific pro-apoptotic functions of IGFBP-3, and suggest that phosphorylation may provide a mechanism for regulation of this action.

Synthesis of insulin-like growth factor binding protein 3 in vitro in human articular cartilage cultures

OBJECTIVE

To quantify the rate of synthesis of insulin-like growth factor binding protein 3 (IGFBP-3) and insulin-like growth factor 1 (IGF-1) by in vitro cultures of normal and osteoarthritic (OA) human articular cartilage.

METHODS

Levels of IGF-1 and IGFBP-3 in media from in vitro cultures of human cartilage were determined by radioimmunoassay (RIA). IGFBPs were characterized by immunoblots and ligand blots. Ultrafiltration and RIA analysis of synovial fluid (SF) samples and washings of cartilage samples ex vivo were used to calculate partition coefficients and to estimate the amount of IGF-1 and IGFBP-3 in cartilage in vivo.

RESULTS

OA cartilage synthesized 150 ng of IGFBP-3 per gm of cartilage per day, compared with 50 ng synthesized by normal cartilage. The surface zone of normal cartilage produced more IGFBP-3 than did the deep zone. Immunoblots and ligand blots confirmed the presence of IGFBP-3. IGFBP-3 synthesis was stimulated by exogenous IGF-1. No freshly synthesized IGF-1 was detected. The quantities of IGF-1 and IGFBP-3 present ex vivo were 11.3 and 78.7 ng/gm of cartilage in normal cartilage and 21.6 and 225.4 ng/gm in OA cartilage.

CONCLUSION

The results show that while IGFBP-3 is synthesized in explant cultures, IGF-1 is not. The rate of IGFBP-3 synthesis is 3 times higher in OA than in normal cartilage. Both IGFBP-3 and IGF-1 penetrate into cartilage from SF in vivo. We estimate that the quantities of IGFBP-3 produced in culture by human cartilage are small compared with the amount supplied in the form of "small complexes" from the circulation. The high value of the partition coefficient of IGFBP-3 implies binding to the matrix.

Isolation and characterization of circulating fragments of the insulin-like growth factor binding protein-3

Proteolysis of insulin-like growth factor binding protein-3 (IGFBP-3), the major carrier of IGFs in the circulation, is an essential mechanism to regulate IGF bioavailability. To analyze naturally occurring IGFBP-3 fragments a peptide library established from human hemofiltrate was screened. Three IGFBP-3 fragments were detected with apparent molecular masses of 34, 16, and 11 kDa. Mass spectrometric and sequence analysis identified the 16 and 11 kDa peptides as glycosylated and non-glycosylated N-terminal fragments spanning residues Gly1-Ala98 of IGFBP-3. Both the circulating forms and those secreted from IGFBP-3(1-98) overexpressing cells bound IGF. Additionally, two smaller fragments (IGFBP-3(139-157) and IGFBP-3(139-159)) were identified in the hemofiltrate. The data indicate that proteolysis of circulating IGFBP-3 occurs in the variable domain at residues alanine 98, phenylalanine 138, glutamine 157, and tyrosine 159.

The amino-terminal region of insulin-like growth factor binding protein-3, (1-95)IGFBP-3, induces apoptosis of MCF-7 breast carcinoma cells

In an earlier study, we reported that an N-terminal proteolytic fragment ((1-95)IGFBP-3) corresponding to the first 95 residues of human insulin-like growth factor binding protein-3 (IGFBP-3) inhibits proliferation in a variety of fibroblasts. With a view to investigating its cytostatic capacity in carcinoma cells, we transiently transfected MCF-7 breast adenocarcinoma cells with an expression vector containing (1-95)IGFBP-3 cDNA. The transfected cells secreted a hyper-glycosylated form of (1-95)IGFBP-3. Twenty-four hours after transfection, cell morphology and viability were similar in control and (1-95)IGFBP-3-secreting cells. However, after 48 h, (1-95)IGFBP-3-secreting cells were apoptotic, with marked cytoplasmic vacuolation and increased free histones in the cytoplasm. Culture media conditioned by (1-95)IGFBP-3-secreting cells also induced morphological changes and apoptosis in wild-type MCF-7 cells, indicating that (1-95)IGFBP-3 was responsible for the effects observed. These results provide further evidence that the N-terminal proteolytic fragment of IGFBP-3 has a functional role.

Interaction Between Acid-Labile Subunit and Insulin-like Growth Factor Binding Protein 3 Expressed in Xenopus Oocytes

The acid-labile subunit (ALS) associates with the insulinlike growth factor (IGF)-I or II, and the IGF binding protein-3 (IGFBP-3) in order to form a 150-kD complex in the circulation. This complex may regulate the serum IGFs by restricting them in the vascular system and promoting their endocrine actions. Little is known about how ALS binds to IGFBP3, which connects the IGFs to ALS. Xenopus oocyte was utilized to study the function of ALS in assembling IGFs into the ternary complexes. Xenopus oocyte was shown to correctly translate in vitro transcribed mRNAs of ALS and IGFBP3. IGFBP3 and ALS mRNAs were injected in a mixture, and their products were immunoprecipitated by antisera against ALS and IGFBP3. Contrary to traditional reports that ALS interacts only with IGF-bound IGFBP3, this study shows that ALS is capable of forming a binary complex with IGFBP3 in the absence of IGF. When cross-linked by disuccinimidyl suberate, the band that represents the ALSIGFBP3 complex was evident on the PAGE. IGFBP3 movement was monitored according to the distribution between the hemispheres. Following a localized translation in the vegetal hemisphere, IGFBP3 remained in the vegetal half in the presence of ALS. However, the mutant IGFBP3 freely diffused into the animal half, despite the presence of ALS, which is different from the wild type IGFBP3. This study, therefore, suggests that ALS may play an important role in sequestering IGFBP3 polypeptides via the intermolecular aggregation. Studies using this heterologous model will lead to a better understanding of the IGFBP3 and ALS that assemble into the ternary structure and circulate the IGF system.

Decreased intracellular degradation of insulin-like growth factor binding protein-3 in cathepsin L-deficient fibroblasts

Proteolysis of insulin-like growth factor binding proteins (IGFBPs) is the major mechanism of releasing IGFs from their IGFBP complexes. Analysis of fibroblasts deficient for the lysosomal cysteine protease cathepsin L (CTSL) revealed an accumulation of IGFBP-3 in the medium which was due neither to alterations in IGFBP-3 mRNA expression nor to extracellular IGFBP-3 protease activity. Incubation of CTSL-deficient fibroblasts with radiolabeled IGFBP-3 followed by subcellular fractionation indicates that both intact and fragmented IGFBP-3 accumulate transiently in endosomal and lysosomal fractions of CTSL-deficient cells. This suggests the involvement of CTSL in the intracellular degradation of IGFBP-3 representing a new mechanism to regulate the extracellular concentration of IGFBP-3.

IGFBP-3 binding to endothelial cells inhibits plasmin and thrombin proteolysis

Insulin-like growth factor-binding protein (IGFBP)-3 contains a highly basic COOH-terminal heparin-binding region, the P3 region, which is thought to be important in the binding of IGFBP-3 to endothelial cells. IGFBP-3 and IGFBP-4, and their chimeras IGFBP-3(4) and IGFBP-4(3), were treated with plasmin and with thrombin, proteases known to cleave IGFBP-3. IGFBP-3 was highly susceptible to plasmin, whereas IGFBP-4 was less so. Substitution of the P3 region for the P4 region in IGFBP-4 (IGFBP-4(3)) increased the ability of the protease to digest IGFBP-4(3); substitution of the P4 region for the P3 region in IGFBP-3 (IGFBP-3(4)) decreased the digestion of IGFBP-3(4). When 125I-labeled IGFBP-3 or 125I-IGFBP-4(3) was first bound to vascular endothelial cells, subsequent proteolysis by either plasmin or thrombin was substantially inhibited. Proteolysis of 125I-IGFBP-3(4) was not inhibited in the presence of endothelial cells. The P3 peptide was cleaved by plasmin but not by thrombin. We conclude that the P3 region is central to proteolysis of IGFBP-3 by plasmin and thrombin, processes which were inhibited by association of IGFBP-3 with endothelial cells.

Mouse testicular Leydig cells express Klk21, a tissue kallikrein that cleaves fibronectin and IGF-binding protein-3

We have cloned a type of cDNA for a functional glandular kallikrein, designated as mouse Klk21 (mKlk21), from the adult mouse testis cDNA library. mKlk21 was expressed in the kidney, submaxillary glands, and testis of the mouse. In the testis, mKlk21 mRNA was detectable at 4 wk of postnatal development and became more prominent thereafter. The mKlk21 gene was expressed exclusively in the Leydig cells of adult mice. When Leydig cells isolated from 2-wk-old mouse testis were cultured in the presence of T, mKlk21 expression was induced significantly. Active recombinant mKlk21 showed trypsin-like specificity, favorably cleaving Arg-X bonds of synthetic peptide substrates. The enzyme activity was strongly inhibited with typical serine protease inhibitors. mKlk21 hydrolyzed casein, gelatin, fibronectin, and IGF-binding protein-3 (IGFBP-3). As in mKlk21, IGF-I and IGFBP-3 were expressed in the Leydig cells of the adult mouse testis, although the transcript of IGFBP-3 was not detected in all of the observed cells. The culture medium of Leydig cells isolated from adult mouse testes contained an mKlk21-like enzyme activity capable of degrading IGFBP-3. These results suggest that mKlk21 plays a role in Leydig cell function in the adult mouse testis.

Mutation of three critical amino acids of the N-terminal domain of IGF-binding protein-3 essential for high affinity IGF binding

The N-terminal domain is conserved in all members of the IGF-binding protein superfamily. Most recently, studies have demonstrated the importance of an IGF-binding protein N-terminal hydrophobic pocket for IGF binding. To examine more critically the amino acids important for IGF binding within the full-length IGF-binding protein-3 protein while minimizing changes in the tertiary structure, we targeted residues I56, L80, and L81 within the proposed hydrophobic pocket for mutation. With a single change at these sites to the nonconserved glycine there was a notable decrease in binding. A greater reduction was seen when both L80 and L81 were substituted with glycine, and complete loss of affinity for IGF-I and IGF-II occurred when all three targeted amino acids were changed to glycine. Furthermore, the ability of the IGF-binding protein-3 mutants to inhibit IGF-I-stimulated phosphorylation of its receptor was a reflection of their affinity for IGF, with the lowest affinity mutants having the least inhibitory effect. These studies, thus, support the hypothesis that an N-terminal hydrophobic pocket is the primary site of high affinity binding of IGF to IGF-binding protein-3. The mutants provide a tool for future studies directed at IGF-dependent and IGF-independent actions of IGF-binding protein-3.

Mutagenesis of basic amino acids in the carboxyl-terminal region of insulin-like growth factor binding protein-5 affects acid-labile subunit binding

Like insulin-like growth factor binding protein-3 (IGFBP-3), IGFBP-5 was recently shown to form ternary complexes with insulin-like growth factor (IGF) and the acid-labile subunit (ALS). Previous studies using IGFBP-5/IGFBP-6 chimeric proteins have identified major and minor ALS binding sites in the carboxyl-terminal and central regions, respectively of IGFBP-5. We now report that ALS binds to IGFBP-3 (K(a) = 1.1 +/- 0.1 liters/nmol) and IGFBP-5 (K(a) = 1.8 +/- 0.5 liters/nmol) with similar binding affinities. Using site-specific mutants, we have identified residues K(211)/R(214)/K(217)/R(218) within the carboxyl-terminal region of IGFBP-5 as being essential for ALS binding. Mutation of K(134)R(136) or K(138)K(139) in the central region of IGFBP-5 resulted in a small decrease in ALS binding.

Characterization of an amino-terminal fragment of insulin-like growth factor binding protein-3 and its effects in MCF-7 breast cancer cells

This study describes the purification, characterization and actions of a peptide derived from proteolysis of IGFBP-3 by an enzyme secreted by MCF-7 breast cancer cells. One millilitre of cell-conditioned medium at pH 5.5 fully proteolysed 10 microg plasma-derived IGFBP-3, yielding an immunoreactive fragment of apparent molecular mass 21 kDa by SDS-PAGE. After purification to homogeneity by IGF-I affinity chromatography and reverse-phase HPLC, sequence analysis revealed the amino-terminus of IGFBP-3, and mass spectrometry indicated a molecular mass of 12 295 Da. Analysis of the corresponding fragment generated by proteolysis of a non-glycosylated IGFBP-3 mutant indicated a molecular mass of 9855 Da, consistent with cleavage after Arg97. This suggests that the fragment derived from glycosylated IGFBP-3 contains approximately 2.5 kDa carbohydrate on Asn89. IGFBP-3[1-97] formed binary complexes with IGFs, but with reduced efficiency compared with intact IGFBP-3. IGFBP-3[1-97] at 11 nM inhibited IGF-I-stimulated DNA synthesis by 50-60% in MCF-7 breast cancer cells, similar to the inhibition observed with the intact protein. In the absence of IGF-I, DNA synthesis was inhibited by IGFBP-3[1-97], but not intact IGFBP-3. This suggests that the IGFBP-3 protease in MCF-7 cell medium can generate an inhibitor of IGF-dependent and independent breast cancer cell growth.

Direct identification of a novel disulfide bond linkage system of new isolated isomer (isomer V) in recombinantly produced h-IGF-I

Insulin-like growth factor I (IGF-I or somatomedin C) is a serum polypeptide with three intramolecular disulfide bonds. In the course of synthesis by the recombinant DNA method, three disulfide bond isomers, all of which have Cys18-Cys61 with three combinations of two disulfide bonds formed by Cys6, Cys47, Cys48 and Cys52, were identified. Natural type, isomer II, was proved to have a Cys6-Cys48, Cys18-Cys61, Cys47-Cys52 disulfide bond system. Now, the fourth isomer, isomer V which doesn't have Cys18-Cys61 disulfide, has been isolated, and its novel disulfide bond linkage system was identified by a chemical synthetic method. The supposed conformation constrained in 3D structure for isomer V would be discussed for its biological activity.

Effect of IGFBP-derived peptides on incorporation of(35)SO(4)into proteoglycans

18 amino acid peptides from the C-terminal region of IGFBP-3, -5 (P3, P5), increased the incorporation of(35)SO(4)into proteoglycans in endothelial cells with greater stimulation in large vessel than microvessel cells. The homologous region of IGFBP-6 (P6) also stimulated sulfate uptake, but less potently than P3 and P5. P6 variants were synthesized with one or two amino acids changed to the basic amino acid in the equivalent position of P3. The P6 variants with one additional basic amino acid behaved similarly to P6. The P6 mutant with two altered amino acids was equipotent to P3. P3F, a scrambled version of P3 was less effective than P3. P3, P5, P6, P3F and all P6 variants all stimulated glucose uptake, which occurred only in microvessel cells. P1, P2, P4, and equimolar intact IGFBP-3 stimulated neither glucose uptake nor sulfate incorporation. Thus, C-terminal basic portions of IGFBP-3, -5 and -6 alter two specific functions of endothelial cells with sufficient differences to suggest mediation by distinct mechanisms.

A purified bovine serum albumin preparation contains an insulin-like growth factor (IGF) binding protein-3 fragment that forms ternary complexes selectively with IGF-II and the acid-labile subunit

Among the six insulin-like growth factor binding proteins (IGFBP), only IGFBP-3 and IGFBP-5 form ternary complexes with IGFs and the acid-labile sunbunit (ALS). In a commercial, highly-purified BSA preparation, ternary complex formation was detected using radio-labeled IGF-II and human serum-derived ALS, with precipitation by ALS antiserum. In contrast, no complexes with radio-labeled IGF-I were detected under the same conditions. Size-fractionation of the BSA on Superose-12 showed the peak of ternary complex forming activity at approximately 30 kDa. To purify the active factor, a solution of the BSA was pumped onto a [Gly(1)]IGF-II affinity column, and eluted fractions were lyophilized and applied to a C18 HPLC column. The eluted fractions showing ternary complex forming activity maintained a preference for IGF-II in forming ternary complexes and a slight preference in forming binary complexes with IGF-II rather than IGF-I. By silver staining after non-reducing SDS-PAGE, the peak activity in the HPLC-eluted fractions appeared as 30 kDa and 21-24 kDa bands. Amino-terminal sequencing of this peak activity revealed bovine IGFBP-3. These results demonstrate that amino-terminal proteolyzed bovine IGFBP-3 is present in a highly purified BSA preparation. In contrast to intact human IGFBP-3 and IGFBP-5, this form of bovine IGFBP-3 forms ternary complexes preferentially with IGF-II rather than IGF-I.

Generation of antisera to mouse insulin-like growth factor binding proteins (IGFBP)-1 to -6: comparison of IGFBP protein and messenger ribonucleic acid localization in the mouse embryo

The insulin-like growth factor (IGF) system is an important regulator of fetal growth and differentiation. IGF bioavailability is modulated by IGF binding proteins (IGFBPs). We have generated six different antisera, directed to synthetic peptide fragments of mouse IGFBP-1 through -6. The specificity of the produced antisera was demonstrated by enzyme-linked immunosorbent assay, Western blotting, and by immunohistochemistry on sections of mouse embryos of 13.5 days post coitum. Specificity for the IGFBP-2 through -6 antisera also was confirmed immunohistochemically in liver and lung of corresponding gene deletion (knock-out) mutant mice and wild-type litter mates. Immunohistochemistry and messenger RNA (mRNA) in situ hybridization on sections of mouse embryos of 13.5 days post coitum revealed tissue-specific expression patterns for the six IGFBPs. The only site of IGFBP-1 protein and mRNA production was the liver. IGFBP-2, -4, and -5 protein and mRNA were detected in various organs and tissues. IGFBP-3 and -6 protein and mRNA levels were low. In several tissues, such as lung, liver, kidney, and tongue, more than one IGFBP (protein and mRNA) could be detected. Differences between mRNA and protein localization were extensive for IGFBP-3, -5, and -6, suggesting that these IGFBPs are secreted and transported. These results confirm the different spatial localization of the IGFBPs, on the mRNA and protein level. The overlapping mRNA and protein localization for IGFBP-2 and -4, on the other hand, may indicate that these IGFBPs also function in an auto- or paracrine manner.

Evaluation of the components of insulin-like growth factor (IGF)-IGF binding protein (IGFBP) system in adolescents with type 1 diabetes and persistent microalbuminuria: relationship with increased urinary excretion of IGFBP-3 18 kD N-terminal fragment

OBJECTIVE

IGFs and their binding proteins (IGFBPs) have an important role in controlling glucose homeostasis and there is evidence to support their involvement in complications related to type I diabetes. The aim of this study was to evaluate the components of the IGF-IGFBP system in adolescents with type 1 diabetes that had developed persistent microalbuminuria (MA).

DESIGN AND PATIENTS

A cohort of 49 adolescents with type 1 diabetes were enrolled in the study. Patients were evaluated at baseline and 1 year later (follow-up). Twenty-six patients with persistent urinary albumin excretion (UAE) of more than 20 microg/min/1.73 m2 (21.6-109. 4 microg/min/1.73 m2) in three different nocturnal urinary collections within 6 months were considered to have MA (baseline mean: 41.9 +/- 22.3 microg/min/1.73 m2; follow-up: 55.9 +/- 24.8 microg/min/1.73 m2). Twenty-three patients with UAE of less than 20 microg/min/1.73 m2 were assigned to the group without MA (baseline mean: 8.6 +/- 3.7 microg/min/1.73 m2; follow-up: 11.8 +/- 4.2 microg/min/1.73 m2). Fasting serum levels of IGFBP-1, IGFBP-2, IGFBP-3, IGF-I and free-IGF-I were determined using appropriate immunoenzymatic, radioimmuno- or immunoradiometric assays. Overnight 12-h urinary collections were obtained and assessed for IGFBP-3 levels, determined by immunoradiometric assay. Urinary and circulating immunoreactive IGFBP-3 forms were determined by Western-immunoblotting (WIB) analysis using a specific polyclonal antibody and monoclonal antibodies directed against N-terminal and C-terminal epitopes of IGFBP-3. IGFBP-3 protease activity was determined using protease assay and by analysis of the intact over the fragmented immunoreactive forms of IGFBP-3 determined by WIB analysis.

RESULTS

Patients with MA showed higher levels of urinary IGFBP-3 (649 +/- 440 ng/h/m2) than patients without MA (398 +/- 229 ng/h/m2; P < 0.05). Urinary levels of IGFBP-3 were directly correlated to UAE (P < 0.001). WIB analysis, using monoclonal antibodies directed against characterized N-terminal and C-terminal IGFBP-3 epitopes, determined that the immunoreactive form of IGFBP-3 found in urine from patients with diabetes was an N-terminal 18 kD fragment. Serum IGFBP-3 levels were lower in patients with MA (baseline: 3613 +/- 598 microg/l; one year follow-up: 3347 +/- 624 microg/l) compared with patients without MA (baseline: 4701 +/- 1484 microg/l; follow-up: 4177 +/- 703 microg/l; P < 0.001). In serum from patients with MA, intact IGFBP-3 was decreased, as indicated by WIB analysis. Conversely, IGFBP-3 proteolysis was increased in patients with MA (baseline: 131 +/- 21% of control; follow-up: 130 +/- 23% of control), compared to patients with normal UAE (baseline: 96 +/- 23% of control; follow-up: 96 +/- 14% of control; P < 0.001). Serum IGFBP-3 protease activity was directly correlated to urinary IGFBP-3 levels (P < 0.001). Serum IGFBP-1 levels were increased in patients with MA (baseline: 36 +/- 20 microg/l; follow-up: 36 +/- 17 microg/l) compared with patients without MA (baseline: 17 +/- 11 microg/l; follow-up: 18 +/- microg/l; P < 0.05). Serum IGFBP-2 levels were also persistently increased in patients with MA (baseline: 503 +/- 134 microg/l; follow-up: 484 +/- 166 microg/l) compared with patients without MA (baseline: 375 +/- 83 microg/l; follow-up: 390 +/- 85 microg/l; P < 0.05). On the other hand, free IGF-I levels were decreased in patients with MA (baseline: 2.3 +/- 1. 5 microg/l; follow-up: 2.5 +/- 1. (ABSTRACT TRUNCATED)

The acid-labile subunit of the serum insulin-like growth factor-binding protein complexes. Structural determination by molecular modeling and electron microscopy

The acid-labile subunit (ALS) is a glycosylated 85-kDa member of the leucine-rich repeat (LRR) protein superfamily and circulates in ternary complexes with the insulin-like growth factors (IGFs) and their binding proteins (IGFBPs). These complexes are thought to regulate the serum IGFs by restricting IGF movement out of the circulation. However, little is known about how ALS binds to IGFBP-3 or -5, which link the IGFs to ALS. To investigate potential sites of interaction, the ALS structure has been modeled with the crystal structure of the LRR protein porcine ribonuclease inhibitor as a template. ALS is predicted to be a donut-shaped molecule with an internal diameter of 1.7 nm, an external diameter of 7.2 nm, and a thickness of 3.6 nm. These dimensions are supported by rotary shadowing electron microscopy of ALS. The internal face is lined with a substantial region of electronegative surface potential that could interact with the positively charged region on IGFBP-3 known to be involved in ALS binding. The model also predicts that three potential N-linked oligosaccharide sites within the LRR domain are clustered together, which may be important in light of recent studies showing ALS glycan involvement in complex formation with IGFBP-3.

Total alanine-scanning mutagenesis of insulin-like growth factor I (IGF-I) identifies differential binding epitopes for IGFBP-1 and IGFBP-3

The bioavailability of insulin-like growth factor I (IGF-I) in the serum and tissues is controlled by members of the IGF binding protein family (IGFBP). These proteins form high-affinity complexes with IGF-I and thereby either inhibit or potentiate its mitogenic and metabolic effects. Thus, understanding the IGF-IGFBP interaction at the molecular level is crucial for attempts to modulate IGF-I activity in vivo. We have systematically investigated the binding contribution of each IGF-I amino acid side chain toward IGFBP-1 and IGFBP-3, combining alanine-scanning mutagenesis and monovalent phage display. Surprisingly, most IGF-I residues could be substituted by alanines, resulting in less than 5-fold affinity losses for IGFBP-3. In contrast, binding of IGFBP-1 was more sensitive to alanine substitutions in IGF-I. The glutamate and phenylalanine at positions 3 and 49 were identified as major specificity determinants for IGFBP-1: the corresponding alanine mutations, E3A and F49A, selectively decreased IGFBP-1 binding by 34- and 100-fold, whereas IGFBP-3 affinity was not affected or reduced maximally 4-fold. No side chain specificity determinant was found for IGFBP-3. Instead, our results suggest that the N-terminal backbone region of IGF-I is important for binding to IGFBP-3. The fact that the functional binding epitopes on IGF-I are overlapping but distinct for both binding proteins may be exploited to design binding protein-specific IGF variants.

The heparin binding domain of insulin-like growth factor binding protein (IGFBP)-3 increases susceptibility of IGFBP-3 to proteolysis

IGFBP-3 proteolysis clears IGFBP-3 from body fluids and increases IGF bioavailability. As shown here, native human IGFBP-3 was cleaved by proteases in media conditioned by hamster and insect cells. This proteolysis was less pronounced for IGFBP-3 containing a mutated heparin binding domain, and was prevented by purifying IGFBP-3 on an IGF-I affinity column in the presence of 2 M sodium chloride, suggesting that the responsible protease(s) binds the IGFBP-3 heparin binding domain. To determine if any human proteases act this way, we first studied plasma prekallikrein since it can copurify with IGFBP-3, and found: 1) [125]IGFBP-3 binds to prekallikrein immobilized either on nitrocellulose or on immunocapture plates; 2) the IGFBP-3 heparin binding domain participates in forming the IGFBP-3/prekallikrein complex; 3) the binary IGFBP-3/prekallikrein complex can bind IGF-I to form a ternary complex; and 4) activation of prekallikrein to alpha-kallikrein by Factor XIIa resulted in proteolysis of bound IGFBP-3. This work suggests: 1) cleavage of IGFBP-3 by a protease may be aided by the ability of the protease zymogen to directly bind the IGFBP-3 heparin binding domain; and 2) direct binding of protease zymogens to IGFBP-3 may explain some instances where IGFBP-3 is preferentially proteolyzed in the presence of other IGFBPs.

IGFBP-3 and IGFBP-5 production by human intestinal muscle: reciprocal regulation by endogenous TGF-beta1

Insulin-like growth factor-I (IGF-I)-mediated growth of cells can be modulated by specific IGF binding proteins (IGFBPs) that inhibit or augment IGF-I ligand-receptor interaction. IGFBP expression and production by human intestinal muscle cells in culture was characterized in rapidly growing cells (day 3 of culture), in confluent cells (day 7), and in postconfluent cells (day 14). RT-PCR analysis identified IGFBP-3, IGFBP-4, and IGFBP-5 mRNA during all three phases of growth. The production of IGFBP-3 and IGFBP-5 was regulated in reciprocal fashion. IGFBP-5 production was high on day 3 and decreased two- to fivefold by day 14, and IGFBP-3 production was low on day 3 and increased five- to eightfold by day 14. IGFBP-4 production remained constant. IGFBP-3 inhibited and IGFBP-5 augmented IGF-I-induced proliferation. IGFBP-3 and IGFBP-5 production was regulated in reciprocal fashion by transforming growth factor-beta1 (TGF-beta1). Immunoneutralization of endogenous TGF-beta1 decreased the production of IGFBP-3 and increased the production of IGFBP-5. Addition of exogenous recombinant human TGF-beta1 had the opposite effect. We conclude that the expression and time-dependent production of IGFBP-3, IGFBP-4, and IGFBP-5 and their regulation by endogenous TGF-beta1 represent mechanisms by which human intestinal muscle cells regulate autocrine IGF-I-mediated growth.

Sequence alterations of insulin-like growth factor binding protein 3 in neoplastic and normal gastrointestinal tissues

Insulin-like growth factor binding protein 3 (IGFBP-3) is an important regulator of normal and malignant cell growth. It modulates the mitogenic effects of insulin-like growth factors (IGFs) by inhibiting growth through mechanisms both dependent on and independent of IGF binding. IGF-I and IGF-II levels are regulated by binding to the IGF-II receptor, which is inactivated by mutation in human gastrointestinal (GI) tumors. We have previously demonstrated elevated IGF-II ligand expression in IGF-II receptor-mutant GI tumors, implicating the IGF signaling system in GI tumorigenesis. Therefore, to investigate the potential involvement of IGFBP-3 in human GI carcinogenesis, direct DNA sequencing of exons 1-4 and intron-exon boundaries of the IGFBP-3 gene was performed in 10 colorectal cancers, 10 gastric cancers, and 10 esophageal cancers. Four distinct sequence alterations were identified: (a) in one gastric and one esophageal tumor, an A to C transversion occurred at nucleotide 5795 (CAC-->CCC), leading to a His-->Pro substitution at codon 179; (b) a second esophageal tumor had a C to T transition at nucleotide 8291 (ACC-->ATC), leading to a Thr-->Ile substitution at codon 277 of IGFBP-3; (c) one alteration comprised a G to C transversion in exon 1 at nucleotide 2132 (GGG-->GCG), leading to a Gly-->Ala substitution at codon 32 in two gastric cancers, seven esophageal cancers, and nine colon cancers; and (d) a C to G transversion located 17 nucleotides from the 3' splice site in intron 1 was observed in three colon cancers and four esophageal cancers. All of these DNA sequence alterations were present in matched normal DNA from the same subjects, which suggests that some or all of them may represent polymorphisms. However, we cannot exclude the possibility that the germ-line nonconservative amino acid substitutions predicted to occur as a result of these alterations result in subtle changes to IGFBP-3 protein function and a predisposition to developing GI malignancy.

Developmental changes in serum levels of free and total insulin-like growth factor I (IGF-I), IGF-binding protein-1 and -3, and the acid-labile subunit in rats

We have recently described a competitive binding assay for rat insulin-like growth factor-binding protein-3 (IGFBP-3) based on the ability of IGFBP-3 to form a ternary complex with the acid-labile subunit (ALS) in the presence of IGF-I. Using this assay we studied groups of male (n = 6) and female rats (n = 6) at 20, 30, 40, 50, 60, 80, and 130 days of age. Nonfasting serum levels of IGFBP-3 were compared with those of total (extractable) IGF-I (tIGF-I) and ALS as well as IGFBP-3 determined by ligand blotting. Additionally, we studied the relationship between ultrafiltered free IGF-I (fIGF-I) and immunoassayable IGFBP-1. IGFBP-3 was dependent on age only (P < 0.0001), but tended to be higher in males than in females (P = 0.06); between 20-130 days levels increased from 6.5 +/- 1.7 to 73.6 +/- 7.2 nmol/liter in males and from 5.4 +/- 1.6 to 51.3 +/- 8.0 nmol/liter in females. IGFBP-3 correlated positively with tIGF-I (r = 0.90; P < 0.0001), ALS (r = 0.92; P < 0.0001), and IGFBP-3, as determined by ligand blotting (r = 0.88; P < 0.0001). The molar ratio of IGFBP-3 to tIGF-I increased from 0.23 +/- 0.04 to 0.76 +/- 0.04 (P < 0.0001) without any sex dependence. An age- and sex-dependent decrease in IGFBP-1 was observed (P < 0.0001), from 10.9 +/- 2.5 to 1.2 +/- 0.2 nmol/liter in females and from 8.9 +/- 0.7 to 0.2 +/- 0.04 nmol/liter in males. Free IGF-I (fIGF-I) increased with age (from 0.7 +/- 0.2 to 7.1 +/- 0.5 nmol/liter; P < 0.0001), and levels were inversely correlated with IGFBP-1 (r = -0.80; P < 0.0001). In young rats, IGFBP-1 circulated in a 10-fold molar excess over the level of fIGF-I, whereas in older rats, fIGF-I exceeded IGFBP-1 by an average of 9-fold in females and by up to almost 60-fold in males. We conclude that in rats 1) IGFBP-3 and fIGF-I are strongly age dependent; 2) IGFBP-3 correlates positively with ALS and tIGF-I; and 3) fIGF-I and IGFBP-1 are inversely correlated. This is in accordance with clinical findings. However, in humans the adult level of fIGF-I rarely exceeds 0.3 nmol/liter, and IGFBP-1 usually circulates in excess of fIGF-I. Thus, our results also imply species differences in the IGF systems of humans and rats.

Influence of treatment with onapristone on the IGF-system in breast cancer patients

The influence of the novel antiprogestin onapristone on the serum insulin-like growth factor (IGF) system was studied in a group of 13 postmenopausal women with metastatic breast cancer. Blood samples were obtained before treatment and subsequently after 1, 2 and 3 months on therapy. IGF-I, IGF-II and IGF-binding protein (IGFBP)-2 were measured by radioimmunoassay (RIA). In addition, the IGFBP profile was evaluated by Western ligand blotting (WLB), and IGFBP-3 fragmentation determined by immunoblotting. A moderate (29%) but significant increase in IGF-I was observed after 3 months on treatment (p < 0.05). IGFBP-2 showed a significant, progressive increase during treatment when evaluated both by WLB (44% increase over baseline at 3 months) and by RIA (33% increase over baseline at 3 months). There was a non-significant trend towards an initial decrease in IGFBP-3 fragmentation. No significant alterations were observed in IGF-II or any of the binding proteins (except IGFBP-2) determined by Western ligand blotting. Due to the observation that onapristone treatment caused a moderate suppression of serum cortisol and androstenedione, we postulate the observed increase in IGF-I to be due to a slight glucocorticoid agonistic effect of the drug. On the contrary, the increase in IGFBP-2 may be related to disease progression as has been observed in patients suffering from prostatic cancer.

Plasminogen binds the heparin-binding domain of insulin-like growth factor-binding protein-3

Limited proteolysis lowers affinity of insulin-like growth factor (IGF)-binding protein (IGFBP)-3 for bound IGFs, resulting in greater IGF bioavailability. Plasmin is one of many proteases that cleave IGFBP-3, and the plasmin system may regulate IGFBP-3 proteolysis and IGF bioavailability in cultured cells in vitro. A role for the plasmin system in IGFBP-3 proteolysis in vivo is suggested by data presented here showing that IGFBP-3 binds plasminogen (Pg; Glu-Pg) with a dissociation constant (Kd) ranging from 1.43 to 3.12 nM. IGF-I and Glu-Pg do not compete for IGFBP-3 binding; instead, the binary IGFBP-3/Glu-Pg complex binds IGF-I with high affinity (Kd = 0. 47 nM) to form a ternary complex. Competitive binding studies suggest that the kringle 1, 4, and 5 domains of Glu-Pg and the heparin-binding domain of IGFBP-3 participate in forming the IGFBP-3/Glu-Pg complex, and other studies show that Glu-Pg in this complex is activated at a normal rate by tissue Pg activator. Importantly, IGFBP-3/Glu-Pg complexes were detected in both human citrate plasma and serum, indicating that these complexes exist in vivo. Binding of IGFBP-3 to Glu-Pg in vivo suggests how Glu-Pg activation can specifically lead to IGFBP-3 proteolysis with subsequent release of IGFs to local target tissues.

Insulin and IGF binding by IGFBP-3 fragments derived from proteolysis, baculovirus expression and normal human urine

Recombinant human IGFBP-3 was proteolysed with different concentrations of plasmin for various periods of time. The major IGFBP-3 fragment resulting from this digestion migrated at ca. 15 kDa in nonreducing SDS-PAGE. Following the identification of this fragment as an N-terminal IGFBP-3 fragment, by use of N-terminus-specific monoclonal antibody and amino acid sequence analysis, we constructed and expressed a similar fragment in a baculovirus expression system. The fragments resulting from plasmin digestion, as well as the baculovirus-expressed recombinant human IGFBP-3(1-97), retain weak IGF binding and show specific insulin binding on cross-linking and western ligand blot. RhIGFBP-3(1-97) can inhibit insulin receptor autophosphorylation in insulin receptor-overexpressing NIH 3T3 cells. Insulin and IGF binding to IGFBP-3 fragments could be further demonstrated in normal urine. These data indicate the physiological significance of IGFBP-3 fragments derived from proteolysis in vivo.

Expression of eukaryotic proteins in soluble form in Escherichia coli

At the optimum temperature for its growth (37 degrees C), Escherichia coli tends to accumulate heterologous proteins in insoluble form. Fusion protein technology has been used to increase the solubility of overexpressed proteins in this organism, but with variable degrees of success. Fusion to a mutant form of DsbA (DsbAmut) confers higher levels of solubility to heterologous proteins in a reproducible way, even when E. coli is grown at 37 degrees C. We have shown this to be true with a diverse sample of eukaryotic proteins: IGF-I, IGFBP-3, 3C proteinase, TGF beta-2, sTGF beta-RII, BDNF, GDNF, mEGFBP, leptin, and GFP. In addition, we have investigated the effects of charge average and proline content on the solubility of DsbAmut fusions. Coexpression of a protein prolyl isomerase [cyclophilin (L-)] and modification of selected asparagine residues to aspartic acid appear to have beneficial effects on the accumulation of soluble heterologous proteins.